Optical wave correlation

ABSTRACT

1. Apparatus for determining the correlation between a message wave and a reference wave which comprises A SOURCE OF LIGHT, A LIGHT-TRANSPARENT COMPRESSION WAVE-SUPPORTING MEDIUM, MEANS FOR ESTABLISHING IN SAID MEDIUM, A TRAVELING COMPRESSION WAVE COUNTERPART OF SAID MESSAGE WAVE, FOR DIFFRACTING LIGHT OF SAID SOURCE, A SPACE PATTERN MASK COUNTERPART OF SAID REFERENCE WAVE DISPOSED BEYOND SAID MEDIUM, A LIGHT-RESPONSIVE DEVICE DISPOSED BEYOND SAID MASK, OPTICAL MEANS FOR PROJECTING THE LIGHT OF SAID SOURCE AS A BEAM THROUGH SAID MEDIUM AND SAID MASK AND ONTO SAID LIGHTRESPONSIVE DEVICE, AND A PARTIALLY TRANSPARENT PHASE-RETARDING MEMBER DISPOSED BETWEEN SAID MEDIUM AND SAID MASK AND IN THE PATH OF SAID BEAM, SAID MEMBER BEING DIMENSIONED TO INTERCEPT AND PARTIALLY TRANSMIT ONLY THE UNDEFLECTED LIGHT OF SAID BEAM BUT TO LEAVE UNAFFECTED LIGHT THAT HAS BEEN DIFFRACTED BY LOCALIZED NONUNIFORMITIES OF DENSITY OF SAID MEDIUM DUE TO TRAVELING COMPRESSION WAVES THEREIN, WHEREBY ALL OF SAID DIFFRACTED LIGHT PASSES AROUND SAID MEMBER.

United States Patent 1 1 3,706,881 Emshwiller 1451 Dec. 19,1972

[54] OPTICAL WAVE CORRELATION a message wave and a reference wave which com- [72] inventor: MacLellan Emshwiller, Orange, NJ. prise a source of llght,

[ g Telcphone Llborllorles, lllcol" a light-transparent compression wave-supporting porated, New York, NY. dium, {22] Fied: March 27 1962 means for establishing in said medium,

a traveling compression wave counterpart of said message wave, for diffracting light of said source, a space pattern mask counterpart of said reference [21] Appi. N0.: 182,758

[52] us. (:1 ..235/181, 250/2l6 Wave disposed beyond said medium- [51] 1111.0. ..G06g 7/19 a "sill-responsive device disposed beyond said [58] Field of Search ..235/l8l; 343/111, 100.7; mask,

33 1 1 J, 145, 1 5; 179/ vs; optical means for projecting the light of said source 329/144; 250/199, 232, 229, 216 as a beam through said medium and said mask and onto said light-responsive device, [56] Reference cu d and a partially transparent phase-retarding member disposed between said medium and said mask and UNITED STATES PATENTS in the path f said beam 3,189,746 6/1965 Slobodin et al ..235/1s1 x said member being dimensicmd ime'cept and partially transmit only the undefiected light of primary Examiner Bem-amin A Borhe1t said beam but to leave unaffected light that has Assistant Binnie] been diffracted by localized nonuniformities of Attorney-R. J. Guenther and William L. Keefauver density of said medium due to tfavefiflg p sion waves therein, whereby all of said diffracted EXEMPLARY CLAIM light passes around said member.

1. Apparatus for determining the correlation between 11 Claims, 5 Drawing Figures PATENTED DEC 19 I972 SHEET 1 OF 2 INVENTOR M. EMSHW/LLER f ATTORNEY PATENTEDuEc 19 I972 SHEEI 2 BF 2 lNVE/VTOR M. EMSHW/LLER NW J A TTORNE V OPTICAL WAVE CORRELATION This invention is concerned with determining the correlation which may exist between message waves. lts object is to increase the speed and improve the certainty with which such determinations may be made.

Broadly, a correlator is an instrumentality which forms the product of two quantities and averages it over a time of interest. More particularly the cross correlation of two time-dependent waves f(t) and g(t) is defined as wherein 7 stands for a phase delay or lag between the two waves. For any specified value of 'r, the cross correlation is merely a number. Advantageously, the correlator develops its output for each of many different values of 'r; i.e., it develops the running cross correlation," itself a time dependent wave.

Evidently, the process is one of comparison between the two waves, which may differ in character and in phase, the degree of similarity being measured by their product. Given that the two waves are generally of the same character, the integrated product is greatest and the match between them is best, when they are in phase coincidence or, in the case of periodic waves, when the magnitude of the lag r is an integral number of full periods. It is this pattern-matching aspect of correlation analysis which is responsible for its great and growing importance in the field of communications technology as expounded, for example, in Application of Correlation Analysis to the Detection of Periodic Signals in Noise" by Y. W. Lee, T. P. Cheatham and .l. B. Wiesner, published in the Proceedings of the Institute of Radio Engineers for Oct., I950, Vol. 38, page 1165.

Instrumentation of these principles by electronic techniques as, for example, in W. R. Bennett et al. U.S. Pat No. 2,676,206 normally requires that all of the various values of r be taken one by one, and the integral evaluated for each of them. To carry out this sequential process takes time, and in some circumstances the time thus lost is prohibitive. This difficulty is avoided by turning to optical techniques; i.e., by passing a beam of light through two wave-pattem masks in tandem, while shifting one relatively to the other. Each mask passes light of intensity proportional to the magnitude of the individual wave which it represents, so that the light falling on a receptor is proportional to their product. An example of this technique is to be found in R. G. Piety U.S. Pat. No. 2,712,415. Because light rays that have traveled along a multitude of different parallel paths through the two masks, and hence represent a multitude of different partial products, can all be gathered together by a lens and directed onto photoreceptor, the apparatus operates with no interposed delay due to sequential operation. Thus, were it not for the time required for the fabrication of the marks, it could operate concurrently with the generation or reception of a message wave to be correlated; i.e., in a real time." This technique is, however, open to a different objection: While a light beam can be extinguished and so brought to zero intensity, its intensity cannot be negative. Hence, without additional complications, the apparatus is restricted to operation with positive magnitudes of the waves to be completed.

The present invention avoids the need for a preformed mask counterpart of the incoming message wave and, at the same time, it escapes from the nonegative-light predicament by imposing the waverepresenting modulations on the light beam as changes of its phase, rather than of its intensity: while the intensity of a light beam cannot be reduced below zero, its phase front can be endowed with an undulating configuration; i.e., advanced and retarded, both locally and temporally, relatively to a mean phase condition, in dependence on the message wave, andsuch advances can be imposed with the same each as such retardations. Accordingly, one of the waves to be correlated, illustra tively an incoming message-modulated carrier wave, is converted into a difi'raction grating which travels across the light beam and causes local departures of the phase from of the light passing through it from its'mean unmodified condition. This traveling grating advantageously takes the form of a transparent compres sion wave-supporting medium or so-called ultrasonic light modulator to the input terminal of which the message wave is applied whereupon, when the medium is terminated to prevent reflection, a compression wave travels from end to end of it, and the localized regions of condensation and rarefaction produce the required phase deviations of the light beam as it passes through them. Light modulators of this variety are disclosed in G. Willard U.S. Pat No. 2,287,587 and in H. L. Barney U.S. Pat. No. 2,451,465. While Willard and Barney employed liquids for their compression wave-supporting media, a solid medium such as quartz is preferred on account of its higher wave propagation speedand lower attenuation.

This traveling wave grating is illuminated by collimated light originating in a. source, and the individual rays that emanate from the various points of the grating are brought to a focus in an image plane. Thus the phase from of the flux of light of which the image is formed carries the same undulation as does that of the light emerging from the wave-supporting medium.

A second fixed grating, also termed mask, prefabricated toembody the major characteristics of the reference wave to be correlated, is fixed in the image plane. Thus the traveling wave grating is imaged on the fixed grating. Evidently, at each instant at which the phase-advanced portions of the image coincide with the transparent portions of the fixed grating, incident light of one phase passes through the latter; while at instants at which phase-retarded portions of the image coincide with the transparent portions of the mask, incident light of a different phase passes through the mask. As the compression waves advance through the wave-supporting medium, the image of the grating which it constitutes travels across the fixed grating, and the phase conditions of the light passing through the mask thus follow one another at the same rate; i.e., at the frequency of the compression wave carrier. Hence a photosensitive device onto which is directed the light that passes through both gratings delivers a current having a substantial component of the carrier frequency.

The prefabrication of the fixed grating, embodying the characteristics of the reference wave, does not interpose delays in the operation of the apparatus as would the prefabrication of a traveling grating embodying the everchanging character of the incoming message wave.

However, the phase deviation (of the light emerging from the wave-supporting medium) from a mean plane wave front, and hence those of its imageon the mask, are small; e.g., not more than a few phase degrees, by far the largest fraction of the light having a mean, undeviated phase. Consequently, the carrier-frequency component of the light fluctuations impinging on the photosensitive device may be largely obscured by the steady component, sometimes accompanied by objectionable noise.

It is therefore advantageous to reduce the intensity of the undeviated light, while leaving enough ofitto make for proper mixing with the deviated light in the photosensitive device, thus to permit recovery of the carrier-frequency component in the output of the latter. For, if the undeviated light were wholly obscured, the principal component of the current delivered by the photosensitive device, whose outputinput characteristic obeys a square law, would be of twice the carrier frequency. From many standpoints, this result would be undesirable.

The invention accomplishes the reduction, short of elimination, of the undeviated light without affecting the phase-deviated light by turning to account the deflection of light rays emerging from the wave-supporting medium which, in accordance with the principles of diffraction, always accompanies the phase deviations of its wave front. The elements of the optical train are proportioned and disposed to form an image, in undeviated light, of the light source in an intermediate plane between the moving grating and the fixed one, and a slightly transparent optical stop is located in this plane having an outline such as to intercept all of the light of this image and of which the optical density is proportioned to transmit a fraction of the incident undeflected light equal to, or slightly greater than, the maximum of the flux of deflected light that passes by it when the wave-supporting medium is energized. In, addition, this partly transparent stop is provided with a film of transparent dielectric material of a thickness and refractive index proportioned to introduce into the undeviated light transmitted by the stop a retardation of the one-quarter wavelength, i.e., 1r/2 radians, is compared with the mean phase of the deviated light which passes around the stop. The effect of this stop is to convert the light incident on the plane of the stop, i.e., light of high intensity, phase-modulated with a low modulation index, into light passing through this plane and incident on the fixed grating which is of low intensity and is modulated in amplitude with a high modulation index.

The resulting light transmitted by the stop, altered in intensity and in phase as described above, serves the same purpose in the demodulation process performed by the photosensitive device, in which the undeviated light is mixed with the diffracted light to give rise to modulation products, as does the unmodulated carrier wave in a radio frequency transmission system. Moreover, and precisely because the phase-modulated light emerging from the wave-supporting medium has been thus converted, as incident on the fixed grating, into light that is modulated in amplitude, both positively and negatively, the fixed grating may safely be of the absorption type without introducing difficulties due to the restriction of its light-passing properties to positive magnitudes.

The invention will be fully apprehended from the following detailed description of an illustrative embodiment thereof taken in connection with the appended drawings in which:

FIG. 1 is a schematic diagram showing cross correlation apparatus in accordance with the invention;

FIG. 2 is a ray diagram of a part of the apparatus of FIG. 1;

FIG. 3 is an enlarged view of partially transparent, phase-retarding optical stop; and

FIG. 4 and FIG. 5 are a phase diagram and a vector diagram, respectively, of assistance in the explanation of the invention.

Referring now to the drawings, the light of lamp 10 is collected by a condensing lens 11 and projected onto a rectangular slit 12 cut in a mask 13. The slit 12 thus illuminated constitutes the light source for the system which follows it. The light emanating from this slit source is collimated by a first lens 1 and directed as a parallel beam onto a transparent compression wavesupporting medium 15 such as water in a suitable container, or quartz. When the medium 15 is undisturbed, the parallel light beam passes through it without modification and to a second lens 2 which converges its rays to a focus in an intermediate plane 16 thus to form, in this plane, an image of the light source 12. An optical slightly transparent stop 17 is disposed in this image plane 16 and of dimensions such that its outline coincides with the source image. It thus intercepts all of the undisturbed light of the source 12. As shown in FIG. 3, this top 17 may be a dense strip formed on a photographic plate 18 that has been exposed, for a suitable time interval, to a flux of light having the same dimensions as the light source 12 and has thereafter been developed. The plate 18 is entirely transparent except throughout the area of exposure of light and, throughout this area, which constitutes the stop 17, its transparency has been controlled through photographic techniques to a small magnitude as more precisely specified below. The plate 18 bears on its rear face a film 19 of transparent dielectric material of a thickness and refractive index proportioned to retard the light rays passing through the stop 17 by one-fourth wavelength as compared with any light rays that may pass around the stop and through the unexposed portions of the plate 18.

The image of the source 12 formed in the intermediate plane 16, attenuated and phase-retarded by the stop 17,19, is collimated by a third lens 3 and directed as a parallel beam onto a mask 21 having the form of a grating of strips which are alternately transparent and opaque or partly so. Like the stop, the mask 21 may be fabricated by photographic techniques. Beyond the grating mask 21 is disposed a fourth lens 4 which accepts those portions of the parallel beam of light that have passed through the transparent portions of the mask 21 and converges them on the photocathode of a photomultiplier tube 23 which may advantageously be provided with a shield 24 to exclude all stray light.

Thus the source image formed in the intermediate plane 16 is, in turn, imaged on the photocathode of the photomultiplier tube 23. When the lenses 1, 2, 3, 4 are all alike, as illustrated, the magnification of the system is unity; the first image of the source 12 formed in the intermediate plane 1 has the same dimensions as the source 12 and the same is true of the second source image formed on the photocathode of the photomultiplier 23. It will be readily apparent to the reader that, by employing lenses of different focal lengths, the magnification of the system may be greater or less than unity without alteration of principle.

The electric current output of the photomultiplier tube 23 is supplied through a bandpass filter 25 to a de tector 26 whose output is, in turn, suppliedto a utilization circuit 27.

The fixed mask 21 is prefabricated to present a space pattern counterpart of a reference wave with which an incoming message wave is to be correlated. Such an incoming wave, having a carrier frequency determined by an oscillator 30 and, through the action ofa modulator 31 carrying information derived from a source 32 in the form of modulations of amplitude or of frequency, is applied as an alternating voltage to the driving terminals 33 of the wave-supporting medium 15. This voltage,'usually through the agency of a piezoelectric crystal driving element, launches compression waves into the medium at the carrier frequency and these travel from the driver end of the medium 15 toward the opposite end at the sound-propagation speed that is characteristic of the medium. At the far end a wave absorber 34 is disposed which prevents reflection and consequent establishment of standing waves. In operation therefore, and at any instant, the medium 15 is characterized by localized regions of condensation and intervening regions of rarefaction, and hence constitutes an optical grating.

The effect of these nonuniformities of density of the medium on the light waves passing through it is illustrated in FIG. 4. Because the light incident on the medium is collimated, the phase front of the emergent light flux when the medium is undisturbed is a plane 40. When the medium 15 is activated, as described above, the phase of the emergent wave is retarded at some points by the greater density of the medium and, at other points, advanced by the reduced density of the medium, giving rise to an undulating phase front 41 of which portions have been retarded in phase while portions b have been advanced. The advance and retardation are small, usually of the order of 1 to With a specified wave-supporting medium and a specified incoming carrier frequency, the wavelength of the pressure waves in the medium is determined. When the carrier is fixed in frequency and is modulated in amplitude the wavelength of the compression waves is uniform from end to end of the medium 15 and the amplitude modulation of the incoming wave finds its counterpart in an amplitude modulation of the compression waves and hence in a variation, from wave to wave, of the amount of condensation and rarefaction which they produce. When, to the contrary, the carrier wave is modulated in frequency but not in amplitude, the condensations and rarefactions produced in the medium are alike in magnitude but are nonuniformly spaced apart.

Whatever be the form of the modulation imposed on the carrier wave, it is advantageous that the fixed mask 21 be prefabricated to duplicate it. Thus, if the carrier wave is frequency-modulated, the mask 2lmay be constituted of opaque strips alternating with transparent strips, nonuniformly spaced apart in conformance with the nonuniform spacing of the density waves in the medium 15. if, to the contrary, the carrier wave is modulated in amplitude, the spacing among the strips of the mask 21 may be uniform while their transparencies and opacities are preferably conformed with the amplitude modulations of the pressure waves in the medium 15 which may be expected to result from any specified amplitude modulation of the carrier wave.

Certain incidental small discrepancies between the mask 21 and the compression wave in the medium 15 which may arise in the course of fabrication may be readily compensated. Thus, for example, in a radio echo-ranging system the carrier which drives the compression wave-supporting medium 15 is not necessarily identical with the radio frequency carrier employed for the location of targets but is preferably of an intermediate freque'ncy, developed by a frequency changing operation. This may be selected at a value such that, for a specified wave-supporting medium 15 and a specified fixed mask 21, the wavelength in the medium 15 conforms exactly to the wavelength of the mask 21, modified only by any magnification that may be found advantageous.

The undulating phase front 41 of the emergent light, developed by the localized regions of condensation and rarefaction in the medium 15 is accompanied by some lateral deflection of this light. In optical terminology, the medium operates as a diffraction grating and forms low intensity satellite images that are displaced to either side of the principal high intensity or zero-order image. These satellite images are formed only in the presence of localized regions of condensation and rarefaction; i.e., they are not formed when the medium is undisturbed. Rather, when the medium is undisturbed, the entire light of the source 12 is focused as a zero-order image on the slightly transparent stop 17. In contrast, when the medium is energized, low intensity satellite images of the source 12 are formed on either side of the zero-order image. The location of these satellite images depends on the wavelength of the density waves in the medium, but not on their amplitude, while the intensity of the satellite images depends on the amplitude of the waves in the medium, but not on their wavelength. Hence the light of these satellite images always passes to one side or the other of the zero-order stop 17 to pass through the third lens 3, the fixed mask 21 and the fourth lens 4 and reach the photocathode of the photomultiplier 23 without attenuation or phase retardation.

Referring to FIG. 5, the large vertical vector represents the magnitude and phase of the undiffracted light passing through the wave-supporting medium 15, i.e., that of the plane wave front 40 of FIG. 4. The undulations of the phase front of the emergent light which arise when the medium is energized and are depicted in the curve 41 are represented as small vectors a and b which extend to the right and to the left, respectively, from the tip of the large vector [0. They represent the intensity and phase of the diffracted light which passes around the stop 17. The resultant is a vector Is, I. of large magnitude that is modulated in phase through a small angle Ad).

,Because of the lateral deflection of the light from which the satellite images are formed, the stop 17 intercepts only the light represented by the large vertical vector l The action of the stop 17, 19 is to reduce the intensity of this light, i.e., the length of the vector and, at the same time, to retard its phase by 90 i.e., to rotate the shortened vector through a right angle. The result of the two operations together is to convert the large vertical vector I into the much shortened horizontal vector 1,. Because the stop l7, 19 has no effect on the diffracted light, the small vectors a and b representing this light may be combined with the new undifiracted light vector I as a and b,, respectively, ofwhich one is in phase coincidence with the vector l while the other is in phase opposition to it. Thus the action of thestop l7, 19 is to convert phase-modulated light of high intensity and a small modulation index. it is well known in the art of transmission of information by carrier techniques that a high index of modulation, short of overmodulation, is of advantage in improving signal-tonoise ratios. The slightly transparent, phase-retarding stop l7, 19 of the invention acts on the light in such a way as greatly to increase the modulation index, and hence serves the same purpose.

In the photomultiplier tube 23 the undiffracted light, attenuated and phase-retarded as described above,is mixed with the diffracted light. Because the outputinput characteristic of the photomultiplier tube 23 obeys a square law, the mixing process gives rise to modulation products of sum frequencies and difference frequencies. Of these a major component is of the frequency of the carrier wave applied to the wave-supporting medium 15. This component is passed by the bandpass filter 25 which is proportioned to have a midband frequency coinciding with the applied carrier frequency and to block the passage of components of other frequencies. The detector 26 which follows the filter 25 recovers the modulation envelope, i.e., that of the message wave delivered by the source 32 and employed to modulate the carrier. This, in turn, is supplied to the utilization circuit 27.

The effect of the correlation technique embodied in the apparatus is, as is now well known, to permit recovery of the desired modulation envelope wave even in the presence of noise in amounts such as largely to obscure the carrier wave and preclude recovery by more straightforward techniques.

To accentuate the advantageous features of the correlation technique, i.e., to achieve maximum precision of congruence between the rays from the traveling grating and the fixed grating, it is advisable, additionally, so to arrange the components of the train of lenses 1, 2, 3, 4 that in addition to imaging the source 12 on the stop 17 and on the photocathode of the photomultiplier 23 as described above, the traveling wave grating developed by the condensations and rarefactions in the wave-supporting medium is, at the same: time, optically imaged on the fixed grating 21. This secondary imaging feature is illustrated in FIG. 2 wherein the second lens 2 and the third lens 3 are individually so disposed, in relation to the traveling wave grating 15 and the fixed grating 21, that the two gratings are located in conjugate planes of these two lenses, taken as a pair. As in the case of the source images, the focal lengths of these lenses may, if desired, be unlike instead of alike in which case the image of the moving grating formed on the fixed grating may be enlarged or reduced in magnitude, as desired.

Advantageously, the apparatus incorporates all of the following features, simultaneously and together:

a. imaging of the light source on the intermediate stop and on the photosensitive device;

b. endowing the stop with transmissivity such as to pass undeflected light of intensity of the same order as the deflected light passed around the stop, and, at the same time, with a dielectric phase retarder;

c. provision of a bandpass filter, tuned to the carrier frequency of the incoming message wave; and

cl. disposition of the lenses to focus on the fixed grating an image of the traveling wave grating.

When employed together, these features permit the employment of a lamp 10 of polychromatic light and a source-slit 12 of substantial dimensions, without defeating the precision of the imaging and correlation techniques. In turn, the large area and broad band of the light source make for a greatly increased light flux incident on the photosensitive device and therefore a greatly increased current available for the utilization circuit.

What is claimed is:

1. Apparatus for determining the correlation between a message wave and a reference wave which comprises a source of light,

a light-transparent compression wave-supporting medium, means for establishing in said medium,

a traveling compression wave counterpart of said message wave, for diffracting light of said source,

a space pattern mask counterpart of said reference wave disposed beyond said medium,

a light-responsive device disposed beyond said mask,

optical means for projecting the light of said source as a beam through said medium and said mask and onto said light-responsive device,

and a partially transparent phase-retarding member disposed between said medium and said mask and in the path of said beam,

said member being dimensioned to intercept and partially transmit only the undeflected light of said beam but to leave unaffected light that has been diffracted by localized nonuniformities of density of said medium, due to traveling compression waves therein, whereby all of said diffracted light passes around said member.

2. Apparatus as defined in claim 1 wherein the optical density of said member is proportioned to reduce the intensity of the undeflected light which passes through said member substantially to the intensity level of the diffracted light which passes by said member.

3. Apparatus as defined in claim 1 wherein said member is proportioned to retard rays passing through it by one quarter light wavelength.

1060M mm 4. Apparatus as defined in claim 1 wherein said member is provided with a film of transparent dielectric material having a refractive index and a thickness proportioned to retard the phase of the undeflected light passing through it by 1r/2 radians relatively to the unretarded light passing by it.

5. Apparatus as defined in claim 1 wherein said optical means are proportioned and disposed to form an image of said source in an image plane between said medium and said mask, and wherein said member is disposed in said image plane.

6. Apparatus as defined in claim 5 wherein said member is of an outline coinciding with said image.

7. Apparatus as defined in claim 5 wherein said optical means are further proportioned and disposed to form a second image of said source on said photosensitive device.

8. Apparatus as defined in claim 1 wherein said optical means are proportioned and disposed to image said wave-supporting medium on said space pattern mask.

9. Apparatus as defined in claim 1 wherein the optical means comprises a first lens, a second lens, a third lens and a fourth lens, the light source being disposed at the entrance focal point of the first lens, thereby to project a collimated beam,

the wave-supporting medium being disposed between the first lens and the second lens in the path of said collimated beam,

the partially transparent member being disposed at the exit focal point of the second lens, whereby the light source is imaged on said member,

said member being also located at the entrance focal point of the third lens, thereby to project a substantially collimated beam, the mask being disposed between the third lens and the forth lens in the path of said last-named collimated beam,

the photosensitive device being disposed at the exit focal point of the fourth lens, whereby the source is substantially imaged on the photosensitive device,

the wave-supporting medium and the mask being further disposed at the conjugate points of said second and third lenses taken as a pair,

whereby the wave-supporting medium is imaged on said mask.

10. Apparatus as defined in claim 1 wherein the optical means comprises a first lens, a second lens, a third lens and a fourth lens,

the light source and the semitransparent member being disposed in conjugate planes of said first and second lenses, taken as a pair,

the semitransparent member and the photosensitive device being disposed in conjugate planes of the third and fourth lenses, taken as a pair,

the wave-supporting medium and the space pattern mask being disposed in conjugate planes of the second and third lenses, taken as a pair.

1]. In combination with apparatus as defined in claim 1,

a bandpass filter connected to said photosensitive device,

said filter having a midband frequency equal to that of said message wave.

1- ugs 

1. Apparatus for determining the correlation between a message wave and a reference wave which comprises a source of light, a light-transparent compression wave-supporting medium, means for establishing in said medium, a traveling compression wave counterpart of said message wave, for diffracting light of said source, a space pattern mask counterpart of said reference wave disposed beyond said medium, a light-responsive device disposed beyond said mask, optical means for projecting the light of said source as a beam through said medium and said mask and onto said lightresponsive device, and a partially transparent phase-retarding member disposed between said medium and said mask and in the path of said beam, said member being dimensioned to intercept and partially transmit only the undeflected light of said beam but to leave unaffected light that has been diffracted by localized nonuniformities of density of said medium, due to traveling compression waves therein, whereby all of said diffracted light passes around said member.
 2. Apparatus as defined in claim 1 wherein the optical density of said member is proportioned to reduce the intensity of the undeflected light which passes through said member substantially To the intensity level of the diffracted light which passes by said member.
 3. Apparatus as defined in claim 1 wherein said member is proportioned to retard rays passing through it by one-quarter light wavelength.
 4. Apparatus as defined in claim 1 wherein said member is provided with a film of transparent dielectric material having a refractive index and a thickness proportioned to retard the phase of the undeflected light passing through it by pi /2 radians relatively to the unretarded light passing by it.
 5. Apparatus as defined in claim 1 wherein said optical means are proportioned and disposed to form an image of said source in an image plane between said medium and said mask, and wherein said member is disposed in said image plane.
 6. Apparatus as defined in claim 5 wherein said member is of an outline coinciding with said image.
 7. Apparatus as defined in claim 5 wherein said optical means are further proportioned and disposed to form a second image of said source on said photosensitive device.
 8. Apparatus as defined in claim 1 wherein said optical means are proportioned and disposed to image said wave-supporting medium on said space pattern mask.
 9. Apparatus as defined in claim 1 wherein the optical means comprises a first lens, a second lens, a third lens and a fourth lens, the light source being disposed at the entrance focal point of the first lens, thereby to project a collimated beam, the wave-supporting medium being disposed between the first lens and the second lens in the path of said collimated beam, the partially transparent member being disposed at the exit focal point of the second lens, whereby the light source is imaged on said member, said member being also located at the entrance focal point of the third lens, thereby to project a substantially collimated beam, the mask being disposed between the third lens and the forth lens in the path of said last-named collimated beam, the photosensitive device being disposed at the exit focal point of the fourth lens, whereby the source is substantially imaged on the photosensitive device, the wave-supporting medium and the mask being further disposed at the conjugate points of said second and third lenses taken as a pair, whereby the wave-supporting medium is imaged on said mask.
 10. Apparatus as defined in claim 1 wherein the optical means comprises a first lens, a second lens, a third lens and a fourth lens, the light source and the semitransparent member being disposed in conjugate planes of said first and second lenses, taken as a pair, the semitransparent member and the photosensitive device being disposed in conjugate planes of the third and fourth lenses, taken as a pair, the wave-supporting medium and the space pattern mask being disposed in conjugate planes of the second and third lenses, taken as a pair.
 11. In combination with apparatus as defined in claim 1, a bandpass filter connected to said photosensitive device, said filter having a midband frequency equal to that of said message wave. 